The conventional method for measuring the barrier properties of films consists of placing a film sample between two chambers containing a test gas of interest in a first chamber and a carrier gas in the second chamber. As the test gas permeates through the film sample, it is collected in the second side and subsequently measured by an appropriate detector. Initially, the measured value of detected test gas will be near zero if the test chambers have been thoroughly flushed of the gas being measured. Over time, the detector response value will increase until it reaches a point of equilibrium, often referred to as the steady state value. This steady state value is then used to define the film material's barrier property in terms of a rate of permeant transmission per unit time per area of measurement. Other factors which contribute to the measurement value are often referenced and include the film thickness, the measurement temperature, relative humidity, and other specifics of the test gas mixture, such as ratios of other gases present.
The relationship among permeability, solubility, and diffusion of the permeant or test gas is normally described by a direct proportionality: P =DS, where P is the permeability coefficient, S is the solubility coefficient, and D refers to the diffusion coefficient. If two of these coefficients are known, the other may be calculated directly. It is apparent, therefore, that an instrument designed to evaluate film protection properties should provide the means to determine at least two of the above coefficients.
Several permeation measurement methods have been described in the literature. The three most common methods include a gravimetric technique, an integrating technique, and a derivative technique. The derivative technique employs a direct measurement of the film permeation value, and since it provides information regarding the transient permeation properties as well as the steady state conditions, this method may also be used to evaluate solubility and diffusion rate values. It is also the method which is most representative of actual end use conditions of the film since the method provides the analysis of permeation from a high concentration to a near zero concentration. However, since the derivative method does not employ a concentration of the measured gas, extremely low senitivities are required, typically on the order of parts per billion. However, such sensitivity has been unobtainable due to the presence of excessive background signal noise which blocks the sensitive readings. It would therefore be of value to the industries involved with materials barrier analysis to have an instrument capable of achieving low sensitivity measurement.
U.S. Pat. No. 5,081,863, issued Jan. 21, 1992 to Reid, teaches an apparatus with multiple test cells for measuring gas transmission through films. The preferred detector of the device of Reid uses a thermal conductivity cell, which is a detector known to be sensitive to temperature changes. This, as well as the multiple cells of Reid, prevents the adequate measurment of a material's barrier performance in the initial stages of transmission, the transition state where signal strength is small.
U.S. Pat. No. 3,902,068, issued Aug. 26, 1975 to Wood, teaches a method of detecting the passage of a gas through a barrier by measuring the variance or modulation of radiant energy transmitted through compressed gas which has passed through the barrier. The presence of modulation of the radiant energy transmitted is indicative of gas passage through the barrier.
U.S. Pat. No. 5,107,696, issued Apr. 28, 1992 to Mayer et at., teaches a system for measuring gas permeability of membranes by using a metallic block possessing exceedingly good heat transfer characteristics which becomes a precisely controlled heat sink for the entire system.